Diesel particulate filters are known in the prior art; examples of which are described in U.S. Pat. Nos. 4,329,162; 4,420,316; 4,416,676; and 6,206,944. Such diesel particulate filters (otherwise referred to as wall-flow filters, diesel particulate traps, honeycomb filters, or exhaust filters) to date have been produced which include a network of porous ceramic walls with a wall thickness of 12-30 mils (305-762 μm) that filter out soot particles from a flow of diesel exhaust. The porous ceramic walls are integrally interconnected to form a matrix of gas-conducting cell channels which typically have a cross sectional shape of square, circular, rectangular, triangular, octagonal, hexagonal, or combinations thereof, for example. The outer region of the matrix of ceramic cell channels may be surrounded by a skin to form a single, unitary structure, which may be cylindrical in shape, for example, but may include any cross sectional shape required for the application.
Such diesel particulate wall-flow filters have an inlet end for receiving exhaust gases, and an outlet end for expelling these gases after being filtered. The cell channels are defined by a plurality of porous ceramic walls which are generally elongated and extend between the inlet and outlet ends. The porous ceramic walls are arranged and configured to typically define a honeycomb structure having a cell density (CD) of between about 10 and 400 cells per square inch (cpsi). The cells defined by the plurality of gas-conducting cells may be plugged at each end in a “checkerboard” pattern, for example; with all inlet channels being plugged at the outlet end, and all outlet channels being plugged at the inlet end. Such plugging of the structure forces the diesel exhaust gases to flow through the porous ceramic walls, thereby filtering out the soot particles generated within the exhaust of diesel engines, hence the term “wall-flow.”
In operation, soot particles in the exhaust gas are trapped on the walls of the inlet channel or in the pores of the wall. As soot particles accumulate on the gas inlet-side of the ceramic walls forming the cells, the pressure drop across (soot loaded back pressure) the filter increases. In some applications, the pressure drop of the filter becomes unacceptably high due to soot accumulation, and the filter must be thermally regenerated. Hence, typically the cells are periodically exposed to conditions initiating a “burnout cycle” designed to ablate the accumulated particles of soot and convert them to ash. Upon completion of the thermal regeneration cycle, the filter is restored again to a generally low back pressure level. Other applications may include a significant amount of passive regeneration such that the soot is, in operation, continuously converted to ash. In either case, management of ash is an important consideration.
For practical applications, a goal for filters is to achieve certain important performance criteria. Among the most important are relatively low pressure drop and relatively high filtration efficiency. Additionally, achieving relatively high strength, and thermal and mechanical durability are also important considerations. Thermal durability, for example, is important since the centrally located ceramic walls of the matrix of a diesel particulate filter may be raised to a very high temperatures (in excess of 800° C.) during the burn out cycles, while the outer skin may only be heated to much lower temperatures. The resulting differential temperature between the matrix core and the skin (sometimes 500+° C.) may create a substantial thermal gradient in the filter. These gradients may create thermal stresses in the particulate filter that may be detrimental. Mechanical durability is also desirable, because filters are subjected to mechanical stresses during manufacture and installation, as well as to applied pressures to the ceramic walls during filter operation. Low pressure drop is desirable to minimize interference with the efficient expulsion of exhaust gases so as to cause a power reduction in the engine. Moreover, in certain applications, it is desirable that the initial filtration efficiency (FE0) be relatively higher to achieve sufficient initial removal of particulate matter from the exhaust gas. This is particularly desirable in filters, which, in operation, undergo a large percentage of passive regeneration. Filters achieving these combinations have proven very elusive. And, although current commercial filters provide an acceptable combination of performance, they have not achieved the superior combinations of properties identified above.
One simple approach to lowering the pressure drop that a filter of a given volume exerts on the exhaust system may be to reduce the thickness of the ceramic walls forming the cells. Such thinner walls would exhibit lower resistance to the flow of exhaust gases passing through the ceramic walls. However, according to previous teachings in the ceramic filter arts, any pressure drop advantage obtained by providing a diesel particulate filter with ceramic walls thinner 12.0 mils would be more than outweighed by the combined disadvantages of lowered mechanical strength, and lowered initial filtration efficiency. Additionally, such thinner walls may also have lowered bulk heat capacity which may result in high temperatures and high thermal gradients during burn out cycles, as well as a propensity for a higher frequency of fuel-expending burn out cycles to ablate the accumulated soot (in active regeneration scenarios) in order to maintain an acceptably low operating pressure drop. This may be especially true in cordierite ceramic materials.
Accordingly, wall-flow filters capable of maintaining a relatively high initial filtration efficiency and relatively low pressure drop have proven elusive. Additionally, achieving such combined attributes of relatively high initial filtration efficiency and relatively low pressure drop, while coupled with also exhibiting sufficient mechanical strength and/or sufficient bulk heat capacity, have, of course, proven even more elusive.